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Alta- you make a good point. After a closer look, perhaps the leaves are opposite, not alternate, which made me think of Parthenium. Few plants have this distinctive flower. If the leaves are opposite, it is likely a Galinsoga, which has this unique flower form AND yellow centers, at least the species seen around here. If stems smooth, G. parviflora, if stems hairy, G. ciliata. My oops.
Dear HeatherY, the plant is native of America, naturalised in almost all parts of world including Himalayas and Kashmir, where I have seen it most, and mentioned in my post. I am presently in California and have spotted it here at many places. See its dstribution:
If you look carefully, USDA has the designation "I" for US and Canada, which is to indicate that the plant is "Introduced", not native.
Just because two continents have a common word in their names does not make them the same place any more than if Africa were named South Eurasia. If a plant is native to Chile, that does not mean it is considered native to the U.S. and/or Canada.
Heather - Please check your plants to see if their leaves are opposite or alternate. If they are alternate, then the plants are likely Parthenium integrifolium, Wild Quinine as I initially proposed. Just noticed in reading today that another common name for Parthenium integrifolium is American Feverfew, in which case you received the seeds you ordered. Check the catalog you ordered from and see what name, if any, they provided beyond "Feverfew".
In regards to where Galinsoga parviflora is native, USDA appears to disagree with itself depending on which site you look at, I think that's where some of the misunderstanding is coming from. This USDA site http://plants.usda.gov/java/nameSearch as greenthumb99 mentions that it's an introduced species in US & Canada, but the GRIN site which singhg45 posted earlier (also connected to the USDA) states that it's native to NM, AZ, and TX http://www.ars-grin.gov/cgi-bin/npgs/html/taxon.pl?104241 Regardless of where it's native though it's got a widespread distribution beyond its native range.